Some general trends in the periodic system

In a level of high angular momentum such as the / level, I = 3, the electron is much closer to the nucleus than the other electrons of the same principal quantum number, as shown by Eq. (22.23). In lanthanum and the succeeding fourteen elements, the rare earths, the high value of I and the low value of the principal quantum number, n = 4, compared to n = 6 in the valence shell both contribute to burying the 4/ electrons in the interior of the atom. As a result, the exterior electrons that give the atom its chemical properties are not much affected by the number of/ electrons present. The chemistries of the rare earths are remarkably similar, differing only by the number of 4/ electrons present. 

7 SOME GENERAL TRENDS IN THE PERIODIC SYSTEM 22.7.1 Atomic Radii 

If we compare the sizes of the atoms in a vertical family of the periodic table such as the alkali metals, we note that the size of the atom increases from the top to the bottom of the column. The valence electron is in the 2s, 3s, 4s, 5s, 6s, 7s level as we pass from lithium to francium. Thus we may retain the general statement that the radius of an atom increases with the principal quantum number of the electrons in the valence shell. The increase is not very rapid. 

In speaking of sizes of atoms, we should keep in mind that the electron cloud does not end at any definite distance from the nucleus. As an experimental quantity the radius of the atom is found to depend a good deal on the environment of the atom during the measurement. The following values for the molecular diameter of argon are obtained by the method indicated  

Method Gas viscosity Liquid density Solid density 

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